The invention relates to a valve arrangement, for example a proportional valve, for controlling the pressure of a connected fluid unit, with a control member which is adjustable by means of a servomotor, counter to a restoring force loading the control member, out of an initial position, in which the fluid unit communicates with a low-pressure reservoir, into an operating position, in which the fluid unit communicates with a pressure source via a connection controlled by the control member and/or is isolated from the low-pressure reservoir.
A valve arrangement of this type, designed as a proportional valve, is known, for example, from German Patent Document DOS 3,144,362.
Corresponding valve arrangements can serve, for example, on vehicles with all-wheel drive, for connecting a connectable driving axle to a main drive train or isolating it from this by appropriately actuating a fluid-controlled clutch.
The capacity for isolating the connectable axle from the main drive train in drive terms is important, above all, when the vehicle has a brake installation with an anti-lock system. In particular, systems of this type can as a rule work effectively only when the vehicle axles are uncoupled from one another in drive terms, that is to say when the speeds of the wheels of one axle are completely independent of the speeds of the wheels of another axle.
Even when the known valve arrangements used in this respect have a high degree of reliability, it is still impossible completely to prevent the possibility that the control member will jam or become sluggish and that the connectable driving axle will accordingly not be isolated from the main drive train or only with a long delay.
A valve arrangement is known from the manual of H. Zoebl "Olhydraulik [Oil hydraulics]", Vienna, Springer-Verlag 1963, page 261, Figure 315, and by means of this a double-acting piston/cylinder unit can, for a particular actuating stroke in one direction or the other, be connected to a pressure source or be shut off form the pressure source, in such a way that the piston is retained immovably in the cylinder. Arranged at the connections of the cylinder chambers in each case are excess-pressure valves which limit the maximum pressure in the piston/cylinder unit. Nevertheless, in the event of a fault of the valve controlling the direction of movement of the piston or its blocking, the piston can remain stationary in a position undesirable per se.
The object of the invention is to guarantee in a valve arrangement of the type mentioned in the introduction an especially high operating safety, and at the same time, in particular, to obtain in response to an appropriate control of the servomotor a pressure relief of the fluid unit with an especially high degree of safety.
According to the invention, this object is achieved in that a passage, which leads from the fluid unit or from the connection controlled by the control member between the fluid unit and pressure source to the low-pressure reservoir, is controllable by means of a safety relief valve, the valve body of which is arranged as a transmission element between the servomotor and control member and is urged into the closing position by the servomotor when the servomotor loads the control member counter to the restoring force.
The invention is based on the general idea of transmitting the actuating force of the servomotor counteracting the restoring force of the control member to the control member via the valve body of the safety relief valve, specifically in such a way that the mutually counteracting actuating and restoring forces are exerted on the safety relief valve with a closing effect. The safety relief valve is consequently opened automatically when, with the control member being jammed or sluggish, the servomotor is actuated with the effect of a return to its position assigned to the initial position of the control member.
According to an especially preferred embodiment of the invention, the passage controlled by the safety relief valve can go through the control member, the valve body of the safety relief valve interacting with a seat of the safety relief valve, this seat being located at the low-pressure end of the passage. A seat-controlled valve of this type can afford an especially high degree of safety against malfunctioning.
There is fundamentally the possibility of connecting the valve body of the safety relief valve to an output member of the servomotor or to design it as part of this, so that, with the control member being jammed or sluggish, the valve body can actively be pulled away from the closing position by the servomotor when the servomotor seeks to return to its position assigned to the initial position of the control member.
Instead, it is also contemplated and advantageous if the valve body of the safety relief valve is arranged separately from the output member of the servomotor and is subjected to the pressure of the fluid unit in the opening direction. Thus, if the control member cannot follow the output member of the servomotor because of jamming or sluggishness, should the latter be returned to its position assigned to the initial position of the control member, the valve body between the control member and servomotor is relieved and pushed open by the pressure of the fluid unit.
As appropriate, the control member can control a plurality of lines which are assigned to different fluid units or different parts of a fluid unit and which, in the initial position of the control member, are all connected to the low-pressure reservoir and, in the event of a displacement of the control member counter to the restoring force, are successively isolated from the low-pressure reservoir and connected to the pressure source, and the passage leading to the low-pressure reservoir branches off from the connection controlled by the control member and, depending on the position of the control member, communicates with one or more of the lines and with the pressure source.
This arrangement takes into account the fact that, even when the control member jams or becomes sluggish, a safety pressure relief is superfluous for those lines which lead to the fluid units and which are already connected to the low-pressure reservoir by the control member. The passage leading to the low-pressure reservoir controlled by the safety relief valve is therefore connected only to the lines or fluid units which communicate with the pressure source in the particular position of the control member.
A valve arrangement of this type, by means of which a plurality of fluid units can be controlled, is suitable, for example, for vehicles with a connectable all-wheel drive and with a plurality of differential locks and affords the possibility first of coupling a connectable driving axle to the main drive train and then, as appropriate, also of successively actuating the lock of a central differential and the lock of axle differentials or successively releasing the actuated differential locks and then also once again uncoupling the connectable driving axle from the main drive train. This uncoupling is thus ensured even when the control member jams or becomes sluggish.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.